init ZigZagConversion project
add test cases for nRows is 1
add test cases for nRows is same as letters number of given string
add test cases for nRows is less than letters number of given string
add test cases for nRows is more than letters number of given string
add test case to fixed a bug

in Python, key word 'is' is using for judging 2 object's id, key word
'==' is using for judging 2 object's value